1. Field of the Invention
The instant invention pertains generally to the field of wireless cellular communication. More particularly, the invention describes a method and apparatus for automatically controlling power level in a backhaul channel of a wireless cellular repeater system.
2. Description of Related Art
In wireless cellular communication systems, it is desirable for a base transceiver station (BTS) to have an indication of the received mobile unit power levels on all the channels in the system at all times. Such information permits the BTS to effectively manage the mobile transceiver units, repeater stations and the home base stations, including actions such as handoff. In repeater based wireless cellular communications systems, any power level measurements for the channels in the system are at best mere approximations.
The reason for such approximations is that in wireless cellular communication systems having repeaters, the signal from the mobile stations is first received by the repeater station. These repeater stations typically have automatic level control (ALC) circuitry that may attenuate or boost the received signal power level to ensure that the signal transmitted by the repeater is received by the serving home base station at a sufficient power level. The ALC alters the characteristics of the received signal including important information such as the received power level. For example, the ALC may cause the power level of the backhaul signal transmitted from the repeater to the base station to be a non-linear function relative to the power received from the mobile units. More specifically, power received from a mobile unit at the repeater station in the range from −25 dBm to −65 dBm would be re-transmitted at −62 dBm in an existing system. In other instances, the ALC may cause the power level of the backhaul signal transmitted from the repeater station to the base station to be compressed in certain power ranges into a ratio of 2:1 for example. More specifically, power received from a mobile unit at the repeater station in the range from −65 to −105 dBm would be re-transmitted at −62 to −82 dBm in an existing system. Under this scenario, a repeater station is difficult to calibrate, test and install.
One of the consequences of automatic level control is that the power level of the signal that is transmitted from the mobile transceiver station to the repeater station is not necessarily correlated to the signal level that is transmitted from the repeater station to the serving home base station. This is because the home base station only receives signal power level information pertaining to the signal that is modified by the ALC in the repeater station. As a result, any indication of the signal power level that the home base station sends to the managing base station controller will not necessarily be an accurate representation of the actual signal power level of the mobile transceiver station or pilot signal from an unused channel. Even if a linear correlation is established between the power level received at the repeater and the corresponding power level received at the base transceiver station, this would still require strict installation procedures and precise backhaul signal configurations to ensure proper power control and handoff functions. There is not much margin for error or tolerance given in such installations. Accordingly, there has been a need to provide a more effective solution to the problem of controlling a wireless cellular communication system having one or more repeaters that would allow for less stringent installation procedures and less performance variations due to radio frequency component characteristics.